Modern techniques for the molecular analysis of cell biology have created an increasing need to prepare samples composed of a homogeneous population of cells. Genomic and proteomic studies, genetic cloning, stem cell studies, and cell-based screening would all benefit from an enhanced ability to obtain living, single cells or small homogeneous biological samples for subsequent analysis. These samples include various molecules such as DNA or RNA as well as cells or organisms.
In the case of selecting cells from a mixed population, individual cells possessing a desired characteristic must be analyzed followed by identification and isolation of a desired subpopulation. Standard sorting methods for mammalian cells require cells to be dispersed in a single-cell suspension, and are most successful with hematopoietic cells which grow naturally in this manner. These methods are less applicable to adherent cells, by far the most common cell phenotype.
Adherent cells are typically analyzed by plating them on a growth surface then looking for them using a microscope. The locations of the cells are random so that finding the cells can be a time consuming process. To speed this up, robotic systems that utilize machine vision are sometimes used to find the cells within the field of view of the microscope image. Traditional sorting techniques for separating cells of interest from a mixed population of cells typically require enzymatic or mechanical release of adherent cells from their growth surface which is detrimental to cell health, or involve extended protocols for selection based on limiting dilution or genetically engineered resistance to a selective environment. In some cases a sacrificial base layer is placed over the plate, cells are grown on the sacrificial base layer and, once cell of interests are found, a subset of the mixed population of cells are isolated by cutting a circle around the cells of interest and through the sacrificial layer with a high powered laser. Cells can be isolated by peeling away the sacrificial layer, or by catapulting the cut material from plate using a high powered laser pulse, carrying the cells of interest with catapulted cut material.
Nonadherent cells can be analyzed quickly using a flow cytometer that rapidly flows a stream of cells past a detector apparatus. Cells of interest can be sorted by a downstream electrostatic system that moves droplets into collection containers. Flow cytometry tends to also work for other biological media such as proteins and DNA if they can be attached to small beads. It tends not to work well for larger samples (such as multi-celled organisms) and is difficult to multiplex.